This invention relates generally to the cooling of turbine blades and, more specifically, to a turbulence-enhancing mechanism on the underside of a tip turn of a cooling passage in a closed-circuit steam-cooled turbine blade.
In a closed-circuit steam-cooled turbine blade, effective cooling of the radially outer tip region is critical for the life of the part. Unlike a typical air-cooled turbine blade where the coolant can be ejected through holes in the blade to provide a relatively cool film over the surface to protect the part, all of the cooling capacity in a steam-cooled turbine blade must come from inside the machine. Heat transfer enhancement, relative to that of a flat plate or pipe configuration, occurs at the tip turns due to the turning of the flow in the serpentine circuit, and is typically sufficient to cool the tip region of the blade under normal operating conditions. However, in the event of a failure in the thermal barrier coating at the tip of the blade, it is desirable to have enhanced heat transfer on the interior surface of the tip in order to maintain metal temperatures below reasonable limits.
The cooling of turbine blade tip regions or tip turns has been discussed in the patent literature. For example, U.S. Pat. No. 6,474,947 describes an air-cooled turbine blade incorporating a configuration of film holes to improve heat transfer in local zones of a typical tip turn. Commonly owned U.S. Pat. No. 5,536,143 relates specifically to the design of a closed-loop, steam-cooled turbine blade and describes the possibility of including turbulence-enhancing features at the tip. More specifically, the patent describes the use of multiple raised ribs of either rounded or square configuration to enhance heat transfer in the tip region. The disadvantage to the use of multiple features, however, is increased pressure loss associated with plural ribs.